Apparatus for implantation into bone

ABSTRACT

An anchor is implantable into a bone in a patient&#39;s body and, when implanted, is resistant to toggling in the bone and to being pulled from the bone. The anchor includes a head end portion having a surface that extends transverse to a central axis of the anchor and is engagable with the bone. A plurality of helical spikes extend from the surface on the head end portion and are engagable with the bone. Each of the plurality of helical spikes has a helical central axis that forms a helix around the central axis of the anchor. Each of the plurality of helical spikes further has a circular cross-sectional configuration as viewed in a plane extending perpendicular to the helical central axis of each of the helical spikes and a distal end portion with a tip that penetrates the bone as the head end portion is rotated relative to the bone.

RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/395,779, filed Mar. 24, 2003 now U.S. Pat. No. 6,953,462, entitled“APPARATUS FOR IMPLANTATION INTO BONE”, which is a continuation-in-partof U.S. patent application Ser. No. 09/708,940, filed Nov. 8, 2000 (nowU.S. Pat. No. 6,551,322), which corresponds to U.S. Provisional PatentApplication Ser. No. 60/238,271, filed Oct. 5, 2000, and which isassigned to the assignee of the present application. The subject matterof the aforementioned application is incorporated herein by reference.

TECHNICAL FIELD

The present invention is directed to an apparatus for implantation intoa bone in a patient's body, and is particularly directed to an apparatusthat, when implanted, is resistant to toggling in the bone and to beingpulled from the bone.

BACKGROUND OF THE INVENTION

Bone screws are used in the medical field for a variety of purposes.Typical uses for bone screws, also referred as bone anchors, includetreating a bone fracture, attaching a corrective device to parts of afractured bone in an area adjacent to the fracture, and attaching acorrective device to a group of bones, such as vertebrae of a spinalcolumn.

Most known bone screws use a conventional screw design, i.e. a solidshank, with one or more external thread convolutions. The solid shankand external threads of the conventional bone screws can cause the bonescrews to displace an undesirably large amount of bone when implanted.It is also known to use a corkscrew-style helical spike as a tissueanchor. The known corkscrew-style tissue anchors, when implanted,displace less bone than the conventional bone screws, but are generallynot able to withstand high tensile loads without structural failure.European Patent No. 0 374 088 A1 discloses a bone screw having atwin-corkscrew design. In this twin-corkscrew design, which is formed bydrilling a passage up through a screw having a solid shank and thenmachining out the material between the two corkscrews, the junction ofthe corkscrews with the shank is unlikely to be capable of structurallywithstanding high tensile loads and repetitive fatigue loads. Thisstructural weakness in the design of the screw in the EP 0 374 088document is further compounded by the corkscrews having a larger overalldiameter than the head of the screw where torque is applied.

One of the more challenging applications of a bone screw is implantationof the screw into the cancellous bone of a patient's spine or pelvis.For example, bone screws are frequently implanted into the cancellousbone of a patient's lumbar vertebrae during a spinal fixation procedureto correct scoliosis. Once implanted, the bone screws are used to mountsuitable spinal fixation instrumentation, such as clamps, rods, andplates. Unfortunately, many of the known bone screws, such as thosedescribed above, can be susceptible to toggling in the vertebral bodyand can also pull out of the vertebral body due to the substantialforces on the screws from human body movement and muscle memory. Inorder to achieve a high pull-out resistance, it is known to thread abone screw all of the way through a vertebrae and place a nut on theopposite side. However, use of such a nut increases the complexity ofthe surgical procedure.

Hence, it is desirable to provide an apparatus for implantation into abone in a patient's body in a minimally invasive endoscopic procedure,wherein the apparatus provides a platform for connecting spinal fixationinstrumentation and, when implanted, is highly resistant to toggling inthe bone and to being pulled out of the bone despite the substantialforces on the apparatus from human body movement and muscle memory.

SUMMARY OF THE INVENTION

The present invention is an anchor which is implantable into a bone in apatient's body and, when implanted, is resistant to toggling in the boneand to being pulled from the bone. The anchor comprises a head endportion having a surface that extends transverse to a central axis ofthe anchor and is engagable with the bone. A plurality of helical spikesextend from the surface on the head end portion of the anchor and areengagable with the bone. Each of the plurality of helical spikes has ahelical central axis that forms a helix around the central axis of theanchor. Each of the plurality of helical spikes further has a circularcross-sectional configuration as viewed in a plane extendingperpendicular to the helical central axis of each of the helical spikesand a distal end portion with a tip that penetrates the bone as the headend portion is rotated relative to the bone.

The present invention further provides a method of implanting an anchorin a patient's body. According to the inventive method, a cannula ispositioned relative to a bone in a patient's body. An anchor, having aplurality of helical spikes extending from a surface on a head endportion of the anchor, is moved along a path extending through thecannula with pointed end portions of the helical spikes leading and thehead end portion trailing. The bone is engaged with the pointed endportions of the helical spikes while moving the anchor along the pathextending through the cannula. The anchor is rotated about a centralaxis of the anchor after engaging the bone with the pointed end portionsof the helical spikes. The bone is penetrated with the helical spikes asthe anchor is rotated about its central axis. The surface on the headend portion of the anchor is moved into engagement with the bone at alocation between the helical spikes.

The present invention further provides an apparatus comprising at leastone anchor which is implantable into a bone in a patient's body and,when implanted, is resistant to toggling in the bone and to being pulledfrom the bone, and a fixation implant for extending between andconnecting a plurality of bones. The at least one anchor includes a headend portion having a surface which extends transverse to a central axisof the anchor and is engagable with the bone. The head end portion hasmeans for connecting with the fixation implant. The head end portion ofthe at least one anchor has a plurality of helical spikes that extendfrom the surface and which extends transverse to a central axis of theat least one anchor and are engagable with the bone. Each of theplurality of helical spikes has a helical central axis that forms ahelix around the central axis of the at least one anchor. Each helicalspike of the plurality of helical spikes further has a distal endportion with a tip that penetrates the bone as the at least one anchoris rotated relative to the bone and a circular cross-sectionalconfiguration as viewed in a plane extending perpendicular to thehelical central axis of the one helical spike throughout a length of theone helical spike extending from the proximal end portion to the distalend portion of the one helical spike.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features of the present invention will becomeapparent to those skilled in the art to which the present inventionrelates upon reading the following description with reference to theaccompanying drawings, in which:

FIG. 1 is a schematic anterior view of an apparatus constructed inaccordance with the present invention implanted in a vertebral body;

FIG. 1A is view similar to FIG. 1 showing the apparatus partiallyimplanted;

FIG. 2 is a schematic anterior view of several vertebral bodiesimplanted with the apparatus of FIG. 1 and connected by a spinalfixation implant in accordance with the present invention;

FIG. 3 is a side view taken along line 3-3 in FIG. 2;

FIG. 4 is an exploded perspective view of the apparatus of FIG. 2, andillustrates a driver for rotating the apparatus;

FIG. 5 is a side view of the apparatus of FIG. 1;

FIG. 6 is a sectional view taken along 6-6 in FIG. 5;

FIG. 6A is a view similar to FIG. 6 illustrating an alternative featureof the invention;

FIG. 7 illustrates an alternate configuration for an end portion of theapparatus of FIG. 1;

FIG. 8 is a side view illustrating a second embodiment of an apparatusin accordance with the present invention;

FIG. 9 is a sectional view taken along line 9-9 in FIG. 8;

FIG. 10 is an exploded perspective view illustrating a third embodimentof an apparatus in accordance with the present invention;

FIG. 11 is a schematic side view of the apparatus of FIG. 10 implantedin a pair of vertebral bodies;

FIG. 12 is a schematic view, partially in section, of a fourthembodiment of the present invention;

FIG. 12A is a view similar to FIG. 12 illustrating an alternateimplantation of the apparatus of FIG. 12;

FIG. 13 is an exploded perspective view of the apparatus of FIG. 12;

FIG. 14 is a schematic posterior view illustrating a fifth embodiment ofthe present invention;

FIG. 15 is a side view of FIG. 14;

FIG. 16 is a sectional view taken along line 16-16 in FIG. 15;

FIG. 16A is a view similar to FIG. 16 illustrating an alternateconfiguration;

FIG. 17 is a side view of a starter tool for use with the presentinvention; and

FIG. 18 is a view showing the starter tool of FIG. 17.

DESCRIPTION OF EMBODIMENTS

The present invention is directed to an apparatus for implantation intoa bone in a patient's body, and is particularly directed to an apparatusthat, when implanted, is resistant to toggling in the bone and to beingpulled from the bone. As representative of the present invention, FIG. 1illustrates an apparatus 10 implanted in a lumbar vertebrae 12. Itshould be understood that the apparatus 10 could be implanted into anynumber of different bones in a mammalian body, including but not limitedto vertebral bones. The lumbar vertebrae 12 has a concave side surface14.

The apparatus 10 comprises an anchor 20 made from a biocompatiblematerial, such as titanium or stainless steel. It is contemplated thatthe biocompatible material used to make the anchor 20 could also bebiodegradable. The anchor 20 is centered about a longitudinal axis 22(FIG. 5). The anchor 20 includes a platform 24 having a generallycylindrical outer surface 26 extending between oppositely disposed firstand second ends 28 and 30 of the platform. The platform 24 includes agenerally rectangular slot 32 that extends axially from the first end 28toward the second end 30 of the platform. Adjacent the first end 28, theouter surface 26 of the platform 24 includes first and second segmentsof external threads 34 and 36 that are separated by the slot 32. Theslot 32 and the threads 34 and 36 provide structure for connectingspinal fixation instrumentation to the platform 24 as discussed furtherbelow. The second end 30 of the platform 24 includes an end surface 38(FIG. 6) having a convex shape that is complimentary to the shape of theconcave side surface 14 of the vertebrae 12. It is contemplated that thecomplimentary shape of the end surface 38 could range from concave toflat to convex depending on the specific shape of the bone surface to beengaged. The end surface 38 of the platform 24 may include barbs (notshown) or other suitable structure for fixedly engaging the side surface14 of the vertebrae 12. As shown in FIG. 6A, the end surface 38 may havea porous texture, formed by mesh, beads, or a coating such as a ceramiccoating, that increases the surface area of the end surface to promotebone in-growth and thus help with long term fixation of the anchor 20 tothe bone.

As shown in FIG. 1A, the platform 24 may further include a central bore39 for receiving a wire (or pin) 21 that has been previously passedthrough the vertebrae 12 across the anatomically defined trajectory. Thewire 21 is used to ensure proper alignment of the anchor 20 duringinsertion into the vertebrae.

First and second helical spikes 50 and 52 project tangentially from theend surface 38 of the platform 24. The helical spikes 50 and 52 resemblea pair of intertwined corkscrews. According to the embodimentillustrated in FIGS. 1-6, the first and second helical spikes 50 and 52extend around the axis 22. The spikes 50 and 52 extend in a helicalpattern about the axis 22 at the same, constant radius R1. It iscontemplated, however, that the first and second helical spikes 50 and52 could extend about the axis 22 at different radiuses. Further, it iscontemplated that the radius of one or both of the first and secondhelical spikes 50 and 52 could increase or decrease as the helicalspikes extend away from the platform 24. In order for the anchor 20 tobe implanted endoscopically through a typical cannula 15 (FIG. 1A), theplatform 24 and the helical spikes 50 and 52 should be less than 20 mmin overall diameter. It should be understood that the anchor 20 couldhave an overall diameter that is greater than 20 mm for certainapplications, and that the anchor could be also implanted in an opensurgical procedure. However, for structural stability reasons, theoverall diameter of the helical spikes 50 and 52 should remain less thanor equal to the diameter of the platform 24.

In the illustrated embodiment of FIGS. 1-6, the first and second helicalspikes 50 and 52 have the same axial length, and also have the samecircular cross-sectional shape. It is contemplated, however, that thefirst and second helical spikes 50 and 52 could have different axiallengths. Further, it is contemplated that the helical spikes 50 and 52could have a different cross-sectional shape, such as an oval shape. Italso contemplated that the first and second helical spikes 50 and 52could have different cross-sectional areas (i.e., one spike beingthicker than the other spike). Finally, it is contemplated that thehelical spikes 50 and 52 should have the same pitch, and that the pitchof the helical spikes would be selected based on the specific surgicalapplication and quality of the bone in which the anchor 20 is to beimplanted.

Each of the first and second helical spikes 50 and 52 can be dividedinto three portions: a connecting portion 54, an intermediate portion56, and a tip portion 58. The connecting portion 54 of each of thehelical spikes 50 and 52 is located at a proximal end 60 that adjoinsthe end surface 38 of the platform 24. The connection portion 54 mayinclude barbs (not shown) for resisting pull-out of the helical spikes50 and 52 from the vertebrae 12. According to one method formanufacturing the anchor 20, the connecting portion 54 of each of thehelical spikes 50 and 52 is fixedly attached to the platform 24 byinserting, in a tangential direction, the proximal ends 60 of thehelical spikes into openings (not shown) in the end surface 38 andwelding the connecting portions 54 to the platform. The insertedproximal ends 60 of the helical spikes 50 and 52 help to reduce tensilebending stresses on the helical spikes under tensile (or pull-out)loads.

Alternatively, the helical spikes 50 and 52 may be formed integrallywith the platform 24, such as by casting the anchor 20. If the anchor 20is cast, it is contemplated that a fillet (not shown) may be added atthe junction of the helical spikes 50 and 52 and the platform 24 tostrengthen the junction and minimize stress concentrations at theconnecting portions 54. The fillet at the junction of the helical spikes50 and 52 and the platform 24 also helps to reduce bending stresses inthe connection portions 54 of the helical spikes under tensile (orpull-out) loads.

As best seen in FIG. 6, the connecting portions 54 at the proximal ends60 of the first and second helical spikes 50 and 52 are spaced 180°apart about the axis 22 to balance the anchor 20 and evenly distributeloads on the helical spikes. The connecting portion 54 of each of thehelical spikes 50 and 52 has a first cross-sectional diameter D1 (FIG.5).

The tip portion 58 of each of the helical spikes 50 and 52 is located ata distal end 62 of the helical spikes. The intermediate portion 56 ofeach of the helical spikes 50 and 52 extends between the tip portion 58and the connecting portion 54. The intermediate portion 56 and the tipportion 58 of each of the helical spikes 50 and 52 has a secondcross-sectional diameter D2 that is less than or equal to the firstcross-sectional diameter D1 of the connecting portions 54. If the secondcross-sectional diameter D2 is less than the first cross-sectionaldiameter D1, the increased thickness of the connecting portions 54 ofthe helical spikes 50 and 52 will help to provide the anchor 20 withincreased tensile strength at the junction of the helical spikes and theplatform 24.

The tip portion 58 of each of the helical spikes 50 and 52 illustratedin FIGS. 1-6 has an elongated conical shape with a sharp pointed tip 68for penetrating into the vertebrae 12 as the platform 24 of the anchor20 is rotated in a clockwise direction. FIG. 7 illustrates analternative, self-tapping configuration for the tip portions 58 whichincludes a planar surface 66 for driving into the vertebrae 12, in thesame manner that a wood chisel turned upside-down drives into wood, asthe platform 24 is rotated. It is contemplated that the tip portions 58could also have a pyramid shape (not shown), similar to the tip of anail.

To implant the anchor 20, a starter tool 600 (FIG. 17) is used to punchtwo holes 602 and 604 (FIG. 18) in the cortical bone of the vertebrae12. The starter tool 600 includes a platform 624 similar to the platform24 and a plurality of helical spikes 650 and 652 similar to the helicalspikes 50 and 52. The platform 624 includes a feature, such as ahexagonal drive projection 630, for drivingly rotating the starter tool600. The spikes 650 and 652 correspond in diameter and quantity to thehelical spikes 50 and 52, but are much shorter in axial length in orderto increase their strength and resistance to radially outwarddeformation. The holes 602 and 604 are punched in locations thatcorrespond to the spacing of the tip portions 58 of the helical spikes50 and 52 on the anchor 20. It should be noted that one or both of theconfigurations of the tip portions 58 illustrated in FIGS. 1-7 may beable to punch through the cortical bone upon rotation of the anchor 20,thus eliminating the need for the starter tool 600 to punch holes in thecortical bone.

As shown in FIG. 18, alignment of the starter tool 600 along the desiredaxis through the vertebrae 12 may be ensured by threading the startertool down over a wire 21 that has been previously passed through thevertebrae. To allow for this, the starter tool 600 may optionallyinclude a central bore 660.

The tip portions 58 are then placed in the holes 602 and 604 in thevertebrae 12 and a rotatable driver 70 (FIG. 4) is inserted into theslot 32 in the platform 24. The driver 70 is then rotated, causing theanchor 20 to rotate as well. It is contemplated that a cylindricalsleeve 17 (FIG. 1A) may be placed around the intermediate portions 56and the connecting portions 54 of the helical spikes 50 and 52 toprevent the helical spikes from deforming radially outward during theinitial rotation of the anchor 20.

Rotation of the anchor 20 screws the helical spikes 50 and 52 into thecancellous bone of the vertebrae 12. The tangentially-orientedconnection between the connecting portions 54 of the helical spikes 50and 52 and the platform 24 minimizes bending loads on the connectingportions during rotation of the anchor 20. Further, thetangentially-oriented connection ensures that the force vector resultingfrom torque and axial force applied by the driver 70 to platform 24 istransmitted along the helical centerline (not shown) of each of thehelical spikes 50 and 52.

As the anchor 20 is rotated, the tip portion 58 of the first helicalspike 50 penetrates the cancellous bone and cuts a first helical tunnel80 (FIG. 1) through the vertebrae 12. Simultaneously, the tip portion 58of the second helical spike 52 penetrates the cancellous bone of thevertebrae 12 and cuts a second helical tunnel 82. The first and secondhelical tunnels 80 and 82 are shaped like the helical spikes 50 and 52,respectively. Continued rotation of the anchor 20 embeds the helicalspikes 50 and 52 deeper into the cancellous bone of the vertebrae 12.The anchor 20 is rotated until the convex end surface 38 of the platform24 seats against the concave side surface 14 of the vertebrae 12 asshown in FIG. 1.

FIGS. 2 and 3 illustrate how the anchor 20 is used for segmental spinalfixation of lumbar vertebrae to treat a patient with scoliosis. Lumbarvertebrae L3-L5, indicated by reference numbers 90, 91, and 92,respectively, are shown in FIGS. 2 and 3. Normally, disk material 94separates each of the lumbar vertebrae 90-92. However, in order tocorrect the scoliosis, the surgeon removes the disk material 94 betweenthe vertebrae 90-92. The spaces left between the vertebrae 90-92 aresubsequently filled with bone graft material 96 (shown schematically inFIGS. 2 and 3) that fuses the vertebrae together over time. Spinalfixation instrumentation, such as a rod or a beam 100, is used tosupport the vertebrae 90-92 until the vertebrae fuse together.

As shown in FIGS. 2 and 3, the vertebrae 90-92 are each implanted withthe anchor 20 according to the present invention as described above. Thebeam 100, which is bent into a desired shape by the surgeon, is placedinto the slot 32 in each of the anchors 20. A nut 102 is then screwedonto the threads 34 and 36 on each of the platforms 24 and is tightenedto secure the beam 100 to each of the anchors 20.

When implanted, the anchors 20 are subjected to substantial forcescaused by human body movement and muscle memory. In some cases, theseforces can tend to pull the known screws used in such an application outof the vertebrae 90-92 or can cause the screws to toggle in thevertebrae. However, when the helical spike 50 and 52 are embedded in thevertebrae 90-92, the two helical spikes of the anchors 20 provide theanchors with a high resistance to pull-out forces. Preliminary cadavertesting indicates that the anchor 20 is so resistant to being pulledaxially from a vertebral body that the vertebral body itself is likelyto fail before the anchor pulls out under high tensile load. Further,the helical spikes 50 and 52, and their tangential connection with theplatform 24, provide the anchors 20 with a high resistance to togglingin the vertebrae 90-92.

FIGS. 8 and 9 illustrate an apparatus 210 constructed in accordance witha second embodiment of the present invention. In the second embodimentof FIGS. 8 and 9, reference numbers that are the same as those used inthe first embodiment of FIGS. 1-6 designate parts that are the same asparts in the first embodiment.

According to the second embodiment, the apparatus 210 comprises ananchor 220 having three helical spikes 230, 231, and 232 projectingtangentially from the end surface 38 of the platform 24. The spikes230-232 extend around the axis 22. As shown in FIG. 9, the connectingportions 54 at the proximal ends 60 of the helical spikes 230-232 arespaced 120° apart about the axis 22, which balances the anchor 220 andevenly distributes loads on the helical spikes. As in the firstembodiment of FIGS. 1-6, in the second embodiment of FIGS. 8 and 9, thecross-sectional diameter of the connecting portions 54 of the helicalspikes 230-232 is greater than or equal to the cross-sectional diameterof the intermediate portions 56 and the tip portions 58 of the helicalspikes.

Each of the three helical spikes 230-232 extends in a helical patternabout the axis 22 at the same, constant radius R1. It is contemplated,however, that one or more of the helical spikes 230-232 could extendabout the axis 22 at different radiuses. Further, it is contemplatedthat the radius of one or more helical spikes 230-232 could increase ordecrease as the helical spikes extend away from the platform 24.

As shown in FIG. 8, the three helical spikes 230-232 have the same axiallength and also have the same circular cross-sectional shape. It iscontemplated, however, that one or more of the helical spikes 230-232could have different axial lengths. Further, it is contemplated that oneor more of the helical spikes 230-232 could have a differentcross-sectional shape, such as an oval shape. It also contemplated thatthe one or more of the helical spikes 230-232 could have differentcross-sectional areas (i.e., one spike being thicker or thinner than theother two spikes). Finally, it is contemplated that the helical spikes230-232 should have the same pitch, and that the pitch of the helicalspikes would be selected based on the specific surgical application andquality of the bone in which the anchor 20 is to be implanted.

The tip portion 58 of each of the helical spikes 230-232 illustrated inFIG. 8 has an elongated conical shape for penetrating into a vertebraeas the platform 24 of the anchor 220 is rotated in the clockwisedirection. It should be understood that the tip portions 58 of thehelical spikes 230-232 of the anchor 220 could alternatively beconfigured like the tip portions illustrated in FIG. 7.

The anchor 220 according to the second embodiment of FIGS. 8 and 9 isimplanted in a vertebrae in the same manner as the anchor 20 accordingto the first embodiment. Further, the anchor 220 according to the secondembodiment may also be used to mount spinal fixation instrumentation insame manner as the anchor 20 according to the first embodiment. Theanchor 220 according to the second embodiment, when implanted in avertebrae, is highly resistant to being pulled out of the vertebrae andto toggling in the vertebrae despite being subjected to substantialforces caused by human body movement and muscle memory.

FIGS. 10 and 11 illustrate an apparatus 310 constructed in accordancewith a third embodiment of the present invention. In the thirdembodiment of FIGS. 10 and 11, reference numbers that are the same asthose used in the first embodiment of FIGS. 1-6 designate parts that arethe same as parts in the first embodiment.

According to the third embodiment, the apparatus 310 comprises an anchor320 having a platform 324. The platform 324 has a threaded outer surface330 adjacent a first end portion 332 and a cylindrical outer surface 340adjacent a second end portion 342. The first end portion 332 of theplatform 324 further includes an axial recess 334. The recess 334 has ahexagonal configuration for receiving a tool (not shown) for drivinglyrotating the anchor 320. The first and second helical spikes 50 and 52project from the end surface 38 of the platform 324.

The apparatus 310 further includes a plate 350 and a nut 360. The plate350 has a first opening 352 for receiving the portion of the platform324 which has the threaded outer surface 330. The plate 350 has a secondopening 354 for receiving a second anchor 320 (see FIG. 11) or otherfixation instrumentation (not shown). When the anchor 320 is implantedin a vertebrae, the nut 360 screws onto the threaded outer surface 330of the platform 324 to secure the plate 350 to the anchor 320.

The anchor 320 according to the third embodiment of FIGS. 10 and 11 isimplanted in a vertebrae in the same manner as the anchor 20 accordingto the first embodiment. FIG. 11 shows a pair of the anchors 320implanted in two cervical vertebrae 370 and 380. The end surface 38 ofeach of the anchors 320 engages a respective anterior surface on each ofthe vertebrae 370 and 380. The plate 350 connects the anchors 320 tohelp support the vertebrae 370 and 380 and transfer loads between thevertebrae until the bone graft material 96 fuses the vertebrae. Like theanchor 20 according to the first embodiment, the anchor 320 according tothe third embodiment, when implanted in the vertebrae, is highlyresistant to being pulled out of the vertebrae and to toggling in thevertebrae despite being subjected to substantial forces caused by humanbody movement and muscle memory.

FIGS. 12 and 13 illustrate an apparatus 410 constructed in accordancewith a fourth embodiment of the present invention. In the fourthembodiment of FIGS. 12 and 13, reference numbers that are the same asthose used in the first embodiment of FIGS. 1-6 designate parts that arethe same as parts in the first embodiment.

According to the fourth embodiment, the apparatus 410 comprises a pairof anchors 420 extending around a longitudinal axis 422. Each of theanchors 420 includes a platform 424 that is substantially wider than theplatform 24 of the anchor 20 in the first embodiment. The platform 424has a cylindrical outer surface 426 that extends between oppositelydisposed first and second end surfaces 428 and 430. An attachment tab440 projects axially away from the first end surface 428 of the platform424. The attachment tab 440 includes a pair of oppositely disposedplanar surfaces 442 and a pair of oppositely disposed arcuate surfaces444.

The attachment tabs 440 provide structure for connecting spinal fixationinstrumentation to each of the platforms 424 and for driving the anchors420. The second end surface 430 of the platform 424 of each anchor 420has a shape that is complimentary to the shape of an upper or lowersurface of a vertebrae. Similar to the first embodiment of FIG. 1-6, theanchors 420 have first and second helical spikes 450 and 452 thatproject from the second end surface 430 of the platform 424. The helicalspikes 450 and 452 extend along the axis 422, but are significantlylarger in diameter than the helical spikes 50 and 52 in the firstembodiment. It should be understood that the anchors 420 couldalternatively have three helical spikes as shown in the secondembodiment of FIGS. 8 and 9.

The apparatus 410 according to the fourth embodiment of FIGS. 12 and 13is particularly useful for a corpectomy application in which a damagedvertebrae is removed. As is shown in FIG. 12, after a portion of adamaged vertebrae 460 is removed, a first one of the pair of anchors 420is implanted into a vertebrae 462 directly above the removed vertebrae460 and a second one of the pair of anchors 420 is implanted into avertebrae 464 directly below the removed vertebrae.

The anchors 420 are implanted in the vertebrae 462 and 464 in much thesame manner as the anchor 20 according to the first embodiment. Arotatable tool (not shown) engages the planar surfaces 442 on theattachment tab 440 and rotates each of the anchors 420 to screw thehelical spikes 450 and 452 of each of the anchors into the respectivevertebrae 462 and 464. The anchors 420 are implanted so that they extendco-linearly along the axis 422. When implanted, the helical spikes 450and 452 of the anchor 420 in the vertebrae 462 extend in an upwarddirection from the platform 430 of the upper (as viewed in the Figures)anchor, while the helical spikes 450 and 452 of the other anchor in thevertebrae 464 extend in a downward direction from the platform 430 ofthe lower (as viewed in the Figures) anchor.

A spinal fixation implant in the form of a cylinder member 480 connectsthe pair of anchors 420 to structurally support the vertebral column inthe absence of the removed vertebrae 460. The cylinder member 480 has acylindrical outer surface 482 and an eccentric inner surface 484. Thecylinder member 480 has a first slot 486 at a first end 488 and a secondslot 490 at a second end 492. The first and second slots 486 and 490receive the attachment tabs 440 on the anchors 420 and allow thecylinder member 480 to be inserted between the anchors. Once insertedbetween the anchors 420, the cylinder member 480 is then rotatedrelative to the anchors about the axis 422. Rotation of the cylindermember 480 brings the arcuate surfaces 444 on the attachment tabs 440 ofthe anchors 420 into frictional engagement with the eccentric innersurface 484 of the cylinder member, thereby securing the cylindermember.

As with the previous embodiments, the anchors 420 according to thefourth embodiment, when implanted, are highly resistant to being pulledout of the vertebrae 462 and 464 and to toggling in the vertebraedespite being subjected to substantial forces caused by human bodymovement and muscle memory.

FIG. 12A illustrates an alternate implantation of the apparatus 410 ofFIG. 12. In FIG. 12A, an apparatus 410′ comprises a pair of anchors 420′that are identical to the anchors 420 described above, except that theyinclude a platform 424′ that has a porous surface 430′, as has beenpreviously described. The porous surface 430′ has an increased surfacearea that promotes bone in-growth. In all other respects, the apparatus410′ is identical to the apparatus 410 described above.

The apparatus 410′ is implanted in the vertebrae 462 and 464 in the samemanner as described above, except that the platforms 424′ on the anchors420′ are recessed into the end surfaces of the two vertebrae, as may beseen in FIG. 12A. The porous surfaces 430′ on the platforms 424′ helpwith long term fixation of the anchors 420′ to the vertebrae 462 and464.

FIGS. 14-16 illustrate an apparatus 510 constructed in accordance with afifth embodiment of the present invention. In the fifth embodiment ofFIGS. 14-16, reference numbers that are the same as those used in thefirst embodiment of FIGS. 1-6 designate parts that are the same as partsin the first embodiment.

According to the fifth embodiment, the apparatus 510 comprises an anchor520 implanted into a sacrum 540. The anchor 520 includes a platform 524having a generally cylindrical outer surface 526 extending betweenoppositely disposed first and second ends 528 and 530. The platform 524includes a slot 532 that extends axially from the first end 528 towardthe second end 530 of the platform. Adjacent the first end 528, theouter surface of the platform 524 includes first and second segments ofexternal threads 534 and 536 that are separated by the slot 532. Theslot 532 and the threads 534 and 536 on the platform 524 providestructure for connecting a rod 550 to the anchor 520.

The second end 530 of the platform 524 includes an end surface 542having a shape that is a complimentary to the shape of a surface 544(FIG. 16) of the sacrum 540. The anchor 520 includes the first andsecond helical spikes 50 and 52 that extend from the end surface 542 ofthe platform 524.

The anchor 520 according to the fifth embodiment of FIGS. 14-16 isimplanted in the sacrum 540 in much the same manner as the anchor 20according to the first embodiment is implanted in the vertebrae 12. Asshown in FIG. 15, in addition to the anchor 520 being implanted in thesacrum 540, known screws 560 are implanted in the pedicles of lumbarvertebrae 562 and 564 above the sacrum. The rod 550 is then bent into adesired shape by the surgeon and placed into the slot 532 in theplatform 524 of the anchor 520. A seat 570 is placed over the first end528 of the platform 524 and engages the rod 550. A nut 572 screws downover the seat 570 and clamps the rod 550 to the anchor 520. In a similarfashion, the nuts 580 secure the rod 550 to the screws 560 implanted inthe vertebrae 562 and 564 above the sacrum 540.

As in the first embodiment, the anchor 520 according to the fifthembodiment, when implanted, is highly resistant to being pulled out ofthe sacrum 540 and to toggling in the sacrum despite being subjected tosubstantial forces caused by human body movement and muscle memory.

FIG. 16A illustrates an alternate construction for an anchor 520′ inwhich the surface 542′ has a porous texture, such as has been previouslydescribed in connection with FIG. 6A. Further, the platform 524 on theanchor 520′ includes a cylindrical outer surface 580 that extendsparallel to the axis of the anchor and has external threads 582 forscrewing into the bone. The anchor 520′ is thus designed to be recessedinto the bone as shown in FIG. 16A to help with fixation as the bonein-growth occurs.

From the above description of the invention, those skilled in the artwill perceive improvements, changes and modifications. It should beunderstood that the present invention can be used for a variety ofpurposes and implanted in other bones besides bones in the vertebralcolumn. Further, it should be understood that more than one of theapparatuses disclosed herein may be implanted into a single bone, suchas a vertebral body. Such improvements, changes and modifications withinthe skill of the art are intended to be covered by the appended claims.

1. A method of implanting an anchor in a patient's body, said methodcomprising the steps of: positioning a cannula relative to a bone in apatient's body; moving an anchor having a plurality of helical spikesextending from a surface on a head end portion of the anchor along apath extending through the cannula with pointed end portions of thehelical spikes leading and the head end portion trailing; forming aplurality of holes in the bone; engaging the bone with the pointed endportions of the helical spikes while moving the anchor along the pathextending through the cannula; rotating the anchor about a central axisof the anchor after engaging the bone with the pointed end portions ofthe helical spikes including moving the pointed end portion of eachhelical spike in the plurality of helical spikes into one of theplurality of holes in the bone; penetrating the bone with the helicalspikes as the anchor is rotated about its central axis; and moving thesurface on the head end portion of the anchor into engagement with thebone at a location between the helical spikes.
 2. A method as set forthin claim 1 further including the step of providing an anchor starterhaving a plurality of helical spikes that correspond in quantity andsize to the plurality of helical spikes on said anchor, the plurality ofhelical spikes on the anchor starter having a short axial length toresist radially outward deformation, said step of forming a plurality ofholes includes rotating the anchor starter to form the holes.
 3. Amethod as set forth in claim 2 further comprising the steps of:positioning a wire through the bone along a desired axis; placing theanchor starter over the wire and sliding the anchor starter toward thebone along the desired axis; engaging the surface of the bone with thehelical spikes on the anchor starter and rotating the anchor starter toform the holes; removing the anchor starter from the wire; placing theanchor over the wire and sliding the anchor toward the bone along thedesired axis; and inserting the helical spikes on the anchor into theholes in the bone formed by the anchor starter.
 4. A method as set forthin claim 2 further comprising the steps of: positioning a wire throughthe bone along a desired axis; and placing the anchor over the wire andsliding the anchor toward the bone along the desired axis.
 5. A methodas set forth in claim 1 further including the step of limiting radiallyoutward deformation of the helical spikes by positioning a sleeve aroundthe helical spikes during rotation of the anchor about a central axis ofthe anchor.